Fossarus multicostatus Pease, 1861a. Length, 5 mm; diameter, 3 mm. Shell: turbiniform; thin; finely spirally striate; white. Spire: protoconch of three and one-half brown whorls, the apical one and one-half whorls smooth, the next with fine axial riblets, the abapical unicarinate and with axial threads; teleoconch of four inflated whorls; suture indistinct. Sculpture: spiral threads between which are weaker spiral striae. Aperture: oblique; base projecting as a moderate canal; neck striated; umbilicus moderately deep. Color: white.
These animals appear to be gregarious, five or more clustering together on the undersurfaces of rocks in shallow water. Beachworn shells are common in drift.
F. multicostatus was described from the Hawaiian Islands but is widely distributed in the Indo-West Pacific, from Reunion and the Persian Gulf to Tahiti.